The invention concerns a biaxial-oriented polyolefin film with improved surface properties, as well as a procedure for manufacturing these films, and its use.
In the plastic treatment industry, in particular in the film branch, surface treatment by means of various pre-treatment methods like e.g. electrical corona discharge, flame or plasma have been part of the prior art for years. These procedures are put to use in order to raise the surface tension, in order to improve metallisability and printability, or rather to increase the compound strength of laminates or adhesive strength of coatings. In corona treatment, a sheet of film is fed over an electrically-earthed support surface, for example a roller. A counter electrode is arranged over this roller at a distance of a few millimeters from the roller. By the application of a mid- to high-frequency AC current, the electrons emitted by the counter electrode ionise the air in the gap between roller and counter electrode. The ions are accelerated by the electric field in the direction of the roller electrode and release their charges and energy onto the film surface. As a result of this bombardment, fragmentation reactions like e.g, separations or chain scissions are initiated. Highly reactive positions in the surface arise, which could react with the reaction products of the corona discharge or could be constituents of the surrounding atmosphere.
These kinds of reactive species can also be generated by other physical procedures, for example by a hot flame (flame treatment) or by discharge procedures in a controlled atmosphere (N2, O2 or Argon) at reduced pressure (low pressure plasma). Most recently, procedures have also been developed in which the plasma is generated e.g. in a nozzle with gas throughflow, and a plasma treatment can be carried out on the substrate surface at atmospheric pressure by alignment of these nozzles.
What all of these procedures have in common is that, by exposure to the reactive species generated, the polymer surface is modified and features improved adhesive properties in the subsequent procedural steps. Procedures working according to this basic principle, as well as devices for carrying out this procedure, are described very extensively with all of their advantages and disadvantages in C. Franz, Kalte Plasmen, Grundlagen, Erscheinungen, Anwendungen, Springer, Berlin 1990 and in A. Grill, Cold Plasma in Materials fabrication from Fundamentals to Application, IEEE Press, N.Y., 1994 and in H. Yasuda, Plasma Polymerization, Academic Press, London, 1985 and in H. Drost, Plasmachemie, Prozesse der chemischen Stoffumwandlung unter Plasmabedingungen, Akademie-Verlag, Berlin, 1978 as well as in the DE-A-27 53 750 (=EP-B-0 002 453, U.S. Pat. No. 4,239,973).
The known procedures have preserved themselves for the raising of the surface tension of films. The activation of the surfaces of films is carried out alternatively in-line, i.e. in the manufacture of the film, or directly before the particular processing step which requires a high surface tension, like e.g. printing, metallisation, lamination. To this end, processors have integrated corresponding pre-treatment stations into their devices. For in-line treatment, the pretreatment stations are integrated into the device for the manufacture of the film, in order to carry out the pre-treatment during the actual film manufacture. Conventionally, they are to be found in the so-called run-out, in order to subject the film to a corresponding treatment after the biaxial stretching and after the heat setting but finally before coiling.
The desired activating effect of a surface treatment is measurable as surface tension of the film. Conventionally, this value for an untreated polypropylene film lies in the range from 27 to 30 mN/m. The surface tension can be raised to 40 to 44 mN/m by corona treatment. This activation of the surface is, however, not boundlessly stable over time. The surface tension of the treated film declines over several weeks, so that after a few months values of 36 to 38 mN/m are obtained, i.e. until treatment of the film comes around the values are so low that before printing, metallisation of other corresponding procedural steps a renewed surface treatment must take place, in order to ensure the desired good adhesive strength. Despite this “refreshment” by a renewed corona or flame treatment, the original maximum values are no longer obtained. Correspondingly, the adhesive properties of these films, or rather the adhesive properties achieved by these procedures, are in need of improvement. According to the composition of the films, these kinds of “refreshments” are sometimes no longer at all possible, or rather lead to undesired side effects due to the ingredients of the films.
The decline of the pre-treatment cannot be improved by higher, i.e. more intensive, in-line pre-treatments as per the prior art, A too-high or too-long pre-treatment damages the film surface, e.g. by mechanical surface roughening or erosion of the surface, and an additionally undesired pre-treatment of the film reverse side (reverse side effect) can come about. Furthermore, high treatment intensities generate an unacceptable characteristic smell (Maggi), which negatively influences the organoleptic properties. Moreover, is was established that higher output values of the surface tension decline comparatively even faster, so that, overall, longer storage times are not possible between manufacture and further treatment.
The retention of the surface tension depends critically on the type of polymers which are subjected to the surface treatment. So, mixed polymerisates, which are made up of various monomers, basically have a longer retention, whereas in homopolymers, in particular propylene homopolymers, the values have already declined so far after three months that the adhesive properties are insufficient. For this reason modifications of the top layer are suggested for the improvement of the adhesive properties.
EP 1282508 describes a modification by a special surface layer of polyolefinic polymers, which contain at least 50% by weight, relative to the weight of the layer, of a maleic acid anhydride modified polyolefin. For the improvement of the adhesive properties, the film is surface treated during manufacture by means of corona or flame after the biaxial stretching. This surface treatment generally declines over time, but when, before the printing, metallisation of lamination a “refreshment” is carried out, the initial values are obtained again because of the modification of the top layer polymers with maleic acid anhydride, whereas with conventional sealing layer or with top layers made of homopolymers at best 80% of the original surface tension is obtained. According to the description in EP 1282508, the second treatment step takes place off-line, i.e. after the manufacture of and storage of the film directly before the further treatment.
Alongside the decomposition of the activated effect over time it is also known that the effect of the surface treatment by raised temperatures is lost. For example, the surface tension of a polypropylene film plasma-treated by means of lower pressure is reduced from 72 to <28 mN/m, if the film is exposed to a temperature of over 130° C. for a few minutes after pre-treatment. This effect prevents a film from being able to be in-line treated before orientation, as this orientation is only possible in the longitudinal and lateral direction at a raised temperature. In practice, for this reason all pre-treatment devices are integrated into the manufacturing devices in such a way that the surface treatment takes place after the biaxial orientation.